Studies will be continued on two highly active enzymes recently reported from this laboratory. Enzyme I was obtained in the serum of dogs, guinea pigs and rabbits by immunization with renin from of kidneys of hog, beef, dog, rabbit and man. Enzyme I produced angiotensin I, optimally, at pH 4,7, at a rate 4000 times higher than the endogenous renin of normal serum. Enzyme II is present in normal serum as an inactive precursor "Proenzyme II," and it is accompained by an "Activating Enzyme" and by an "Inhibitor" of the activating enzyme. The inhibitor has been separated by acid-treatment and fractionation of dog serum. Proenzyme II, thereafter, can be activated enzymatically to Enzyme II at 38 degrees C or by storage in the frozen state. Enzyme II produces angiotensin I optimally pH 4.7 at a rate 2300 times higher than the endogenous renin in the serum. Future studies will be concerned with the purification and identification of Enzymes I and II which are not identical with renin, cathepsin D, pepsin or plasmin. Investigations are contemplated also on the substrate of Enzyme II which differs from that of Enzyme I and renin. The inhibitor of the activating enzyme is present in normal serum of dog, hog, beef and man; attempts will be made for isolation and identification. Studies will be carried out also with other specific inhibitors (pepstatin, trasylol, diisopropyl fluorophosphate, soybean trypsin inhibitor, etc.) to identify or classify these three, unknown enzymes.